1. Field of the Invention
This invention relates to a catalyst for the high-temperature steam reforming of hydrocarbons and more particularly to such a catalyst which exhibits high activity and a long catalytic life and precipitates less carbon even when used particularly under a comparatively low pressure and a low steam/carbon ratio.
2. Prior Art
The high-temperature steam reforming of hydrocarbons is carried out as a process for producing hydrogen by reacting hydrocarbons with steam at high temperatures and, in the operation of the process, a temperature at the inlet of the catalyst bed and a temperature at the outlet thereof are 500.degree.-600.degree. C. and 700.degree.-850.degree. C., respectively. In this process, there are obtained carbon monoxide, carbon dioxide and a minute amount of methane. Catalysts which have heretofore been used in such a process for the high-temperature reforming of hydrocarbons, are Ni-based ones containing an alkali metal or alkaline earth metal. An ordinary high-temperature steam reforming reaction is carried out under a comparatively high pressure (20 Kgf/cm.sup.2 or higher) and a high steam/carbon ratio (3.0 or higher), but as low a reaction pressure as possible is preferred for a fuel cell system in which gases obtained by the steam reforming of naphtha or kerosene are used as a fuel, from the standpoint of apparatus within said system being easily handled. Further, it is preferable to carry out steam reforming at as low a steam/carbon ratio as possible from the viewpoint of electricity generation efficiency. If, with respect to the fuel cell, an ordinary Ni-based catalyst is used in steam reforming which requires a low pressure and a low steam/carbon ratio, there will be raised a serious problem as to the precipitation of carbon on the reforming catalyst. In a case where carbon precipitation on the catalyst is remarkable, the catalyst bed will be blocked with the result that the pressure drop at the catalyst bed (difference between the pressures) is increased and further continuous operation of the apparatus comes to be impossible as far as the catalyst is not exchanged by new one. In addition, frequent exchange of the catalyst will remarkably aggravate the economy of the fuel cell system. It is therefore desirable to use, as a steam reforming catalyst, a catalyst which will precipitate carbon in as small an amount as possible when used.
As such catalysts as precipitate less carbon, rhodium- or ruthenium-based catalysts have been known to be particularly effective. For example, Japanese Pat. Appln. Laid-Open Gazettes Nos. Hei 2-43950 (43950/90), Hei 2-2878 and Hei 2-2879 as well as Sho 56-91844 (91844/81) propose noble metal-based catalysts carried on zirconia. Even these proposed catalysts will not be able to sufficiently inhibit carbon precipitation if they are used under a low pressure and a low steam/carbon ratio.
Further, Japanese Patent Gazette No. Sho 59-29633 (29633/84) describes that a ruthenium catalyst in which ceria is used as an adjuvant catalyst is effective for a low-temperature steam reforming reaction of hydrocarbons, but this catalyst is still not sufficiently effective for a high-temperature steam reforming.